Looks like the 1/8 Key is used on 1/4-20 or 1/4-28 (fine pitch) setscrews...That seems large.
Can you remove the other setscrew and take some measurements? I don't know if that is easy or a nightmare...
Can you get the length? I may have some in my junk box. Otherwise try going to Home depot, they may have some in those Kit drawers...